1. Field of the Invention
The present invention relates generally to a mobile communications system, and in particular, to transmitting and receiving high-speed shared-control channel (HS-SCCH) information in a mobile communications system supporting a high-speed downlink-packet access (HSDPA) scheme.
2. Description of the Related Art
Code-Division Multiple-Access (CDMA) systems allow many users simultaneously to access a given frequency allocation. User separation at the receiver is possible because each user spreads its respective modulated data waveform over a wide bandwidth using a unique spreading code (also referred to as a “channelization code”), prior to transmitting the waveform. Such spreading typically involves, e.g., multiplying the data waveform with a user-unique high-bandwidth pseudo-noise binary sequence. At the receiving end, the receiver re-multiplies the signal with the pseudo-noise binary sequence to remove substantially all of the pseudo-noise signal, so that the remaining portion of the signal is just the original data waveform. Ordinarily, users spread their signals using codes that are orthogonal to each other, i.e., do not interfere with one another. However, a common problem is inter-symbol interference (ISI), i.e., distortion of a received signal typically manifested in the temporal spreading and consequent overlap of individual pulses from users who are physically proximate to one another to the degree that a receiver cannot reliably distinguish between changes of state representing individual signal elements. ISI can present a significant problem if the power level of the desired signal is significantly lower than the power level of the interfering user (e.g., due to distance) and, at a certain threshold, can compromise the integrity of the received data.
One technique for handling ISI is the use of equalizer-based receivers, which are a promising technology for high-speed data transmission systems, such as systems that conform to the High-Speed Downlink Packet Access (HSDPA) standard, which is part of the Third-Generation Partnership Project (“3GPP,” or simply “3G”). Equalizer-based receivers typically use linear channel equalizers to restore the orthogonality of spreading sequences lost in frequency-selective channels (i.e., channels for which the propagation is a strong function of frequency within the bandwidth of the channel), thereby suppressing ISI, such as might occur in a downlink operating under the Wide-Band CDMA (WCDMA) standard (a 3GPP technology). Equalizer-based receivers also have the advantage of being of relatively low complexity for short to moderate signal-delay spreads. In addition to equalizer-based receivers, rake receivers are also used with HSDPA systems.
Due to the fast evolution of the mobile communications market, a major increase in demand for data traffic and high bit-rate services is taking place. To meet this demand, systems are increasing their spectral efficiency (i.e., the amount of information that can be transmitted over a given bandwidth in a specific digital communication system) and are supporting increasingly higher user-data rates, particularly in the downlink direction of the communications path, due to its heavier load relative to the uplink direction.
One technique used in HSDPA for increasing 3G data rates is the use of shared-channel transmission, whereby a certain portion of the channelization codes and transmission power in a cell are considered a common resource that is dynamically-shared among users, primarily in the time domain. Shared-channel transmission makes more efficient use of available code resources than standard WCDMA networks that employ only dedicated channels, which are logical channels allocated to only individual users. The increased efficiency of code and power use can boost cell capacity by more than twice that of a dedicated channel in a standard WCDMA network, thus enabling higher data rates. Shared-channel transmission in HSDPA is accomplished using a High-Speed Downlink Shared Channel (HS-DSCH), which is a downlink data channel for supporting high-speed transmission of downlink packet data, together with its associated control channels. One such control channel is a High-Speed Shared Control Channel (HS-SCCH), which carries downlink information necessary for HS-DSCH demodulation.
Each frame (or data block) transmitted over an HS-SCCH channel has a three-slot duration corresponding to a Transmission Time Interval (TTI) of approximately 2 ms. This frame is divided into two functional parts: The first slot (part 1) carries the time-critical information that is needed to start the demodulation process in time to avoid chip-level buffering, i.e., storing data as chips in a buffer, which typically employs on the order of one thousand times the amount of storage needed to store the original data prior to coding and transmission as symbols, i.e., at the symbol level. The next two slots (part 2) contain less-time-critical parameters, including (i) Cyclic Redundancy Check (CRC) to check the validity of the HS-SCCH information and (ii) process information for Hybrid Automatic-Repeat Request (HARQ), which is an operation designed to reduce the delay and increase the efficiency of re-transmitting data. For protection, both HS-SCCH parts employ terminal-specific scrambling (or “masking”) to allow each terminal to decide whether the detected control channel is actually intended for that particular terminal.
In an HSDPA implementation, the Universal Terrestrial Radio-Access Network (UTRAN) allocates a particular number of HS-SCCH channels that corresponds to the maximum number of users that will be code-multiplexed in the network. From the network point of view, there may be a large number of HS-SCCH channels allocated, but each terminal (also referred to as “User Equipment” (UE)) will need to monitor and consider a maximum of only four HS-SCCH channels at a given time. The four channels can be simultaneously intended for up to four different UEs, where a UE cannot have more than one channel intended for it.
A UE has a relatively short duration—e.g., only a single slot—to determine which codes to despread from the HS-DSCH channel. A single UE monitors and considers a maximum of four HS-SCCH channels (i.e., part 1 of the HS-SCCH frame of each channel). If the UE detects a positive indication on one of the four control channels, i.e., that data intended for the UE is being transmitted on that channel, then the UE monitors only that channel in the consecutive TTI, which is done to increase UTRAN signaling reliability. However, part-1 detection is not as reliable as part-2 detection, during which CRC checking is performed. Thus, disadvantageously, false detection of an HS-SCCH channel can occur when part-1 decoding provides a positive indication, and later, the part-2 CRC check fails. Such false detection will not only trigger unnecessary despreading and decoding of the HS-DSCH data channel, but more significantly, will mislead the UE to monitor only one HS-SCCH channel in the next TTI. Accordingly, a high false-detection ratio during HS-SCCH channel decoding can have an impact on throughput performance and require unnecessary physical-layer processing that increases UE power consumption.
Moreover, the false-detection ratio of HS-SCCH part 1 based on Viterbi state metrics is undesirably high, particularly when the received-signal level is good. To reduce false detection of HS-SCCH part 1, a predefined correlation threshold used in the decoding process can be increased. However, increasing the correlation threshold undesirably causes missed detection of data actually intended for the UE when the received-signal level is poor. Therefore, HS-SCCH detection based on Viterbi state metrics alone implies a trade-off between false detection and missed detection.